In vision based automated sorting systems, such as that used for sorting fruit and vegetables, the product to be sorted is normally discharged off the end of a horizontal conveyor belt. In the sorting process, the product is optically scanned while on the conveyor belt and/or while in flight off the end of the belt. An accept/reject decision is made based on the outcome of the optical scanning, and if appropriate, the product is rejected by a rejection device deflecting it out of its normal trajectory into a reject chute.
For large objects, such as whole fruits or vegetables or similar sized foreign materials (stones, earth clods etc.) mechanical rejection is most suitable for deflecting the rejected objects into the reject chute. The rejection device is commonly made up of a bank of mechanical reject actuators such as fingers or paddles or boppers arranged across the width of the conveyor. Typically mechanical reject actuators are spaced at a pitch of 25 mm (or 1″) across the width of the conveyor carrying the incoming product stream. When an accept/reject decision is made based on the outcome of optical scanning, a signal is sent to one or more of the mechanical reject actuators extended across the width of the conveyor. In response to this the relevant mechanical reject actuator will activate and eject the product from the in flight stream. Pneumatic/mechanical rejection actuators such as that disclosed in EP1 605 170 are commonly used.
For small objects such as diced tomato or peppers, dried prunes etc. or similar sized foreign materials (small vines, twigs, leaves etc.) it is often more appropriate to deflect these into the reject chute using air jets. A line of air nozzles is normally arranged on a bar across the width of the conveyor and each nozzle can output an air jet in response to the accept/reject decision based on the outcome of the optical scanning.
Mechanical and air jet ejectors are commonly used, not only in the automated sorting of fruit and vegetables, but also in waste recycling and separation/sorting of solid raw materials such as mineral ores etc.
Selecting the type of rejection system to be installed (mechanical or air jet) based on the type of product being sorted is generally sufficient when the objects to be rejected are similar in shape and size to the product (e.g. actual products with undesirable quality defects). However, in practical applications the undesirable objects that need to be rejected will cover the whole spectrum of sizes from twigs and leaves to large stones or small animals (e.g. rabbits).
Mechanical rejecters can be employed to handle the larger objects, however with lighter smaller objects the mechanical rejecters will tend to miss them or deflect them only weakly and so not positively separate them from the acceptable product. Plastic bags, for example, may not be deflected sufficiently and may become wrapped around the mechanical reject actuator.
Air jets, on the other hand, are ideal for deflecting small light objects but will not have sufficient power to divert large heavy objects—unless excessive volumes of air are used. large volumes of air. In general it is preferable to use mechanical rejection rather than air rejection as typically rejecting an object with free air from nozzles consumes more energy compared to converting the air energy into mechanical energy in the confined spaces of a pneumatic/mechanical rejection actuator.
This usually means that several stages of sorting are carried out to remove such unsuitable objects prior to the main sort (e.g. vine removers, graders, stone/clod removers, leaf blowers etc). As well as the economic cost of requiring additional machinery, these processes involve additional handling of delicate food products potentially resulting in damage, reduced quality or reduced yield.